The subject of the present invention is a sole for footwear which is curved into the overall shape of a foot or intended to be adapted to the particular shape of a foot.
It is known practice to use insoles for footwear comprising a surface essentially corresponding to that of the bottom of the item of footwear and possibly fitted with localized support and/or shock-absorbing elements. Such soles may be comfort soles or alternatively may be soles intended to improve the stability of the foot or to protect the bone structure against repetitive shock, particularly for use in sport, such as walking or running.
The sole according to the invention, which may be an insole or an outer sole for footwear, is aimed more specifically at stabilizing the lateral movement of the foot and at assisting and guiding it in its longitudinal movement during walking, running or when participating in other sports.
When walking or participating in some sport, a foot unfurls its various pressure points almost systematically from the outside of the heel forward, passing via the lateral arch and the front of the foot to finish at the big toe. The path of the barycentres in the dynamics of walking describes a curve in the overall shape of an S from the rear forwards and between the outside of the heel and the first inter-metatarsal space.
The speed of walking, just like the speed of running, depends above all on the duration of the pressure phase.
The pressure time depends on the load-absorbing and energy restitution capacity of the anatomical structures. This restitution of energy is associated with the elasticity of the tissue. Now, most chronic lesions are due, above all, to the significant forces involved in the load-bearing phase. Furthermore, the kinetic energy generates sheer forces at the surface of the skin, and these are likely to give rise to lesions. Most chronic lesions do not occur during the landing and shock-absorbing phase but during the load-bearing phase, at the moment when the foot absorbs the load and flattens by pronation, and when it has to restore the energy through elasticity of the tissue and muscular contraction.
The object of the invention is to provide a sole which stabilizes the lateral movement of the foot and guides the latter in its longitudinal movement so as to increase the elastic reserve of the foot.
To this end, the sole to which it relates, of the aforementioned type, comprises a strengthener in the form of a closed loop which surrounds the zone on which the heel rests and is extended forward in the form of two branches extending along the two edges of the sole at least as far as the zone of the first and fifth metatarsal heads. This strengthener may form an integral part of the sole or may be attached under the sole and secured to the latter.
The loop-shaped part of the strengthener surrounding the heel is intended to stabilize its lateral movement upon contact with the ground. Specifically, eversion or inversion thereof during this phase leads to the unfurling of the step shifting off-line during the pressure and propulsion phases, leading to a loss of energy but also, and above all, to risks of trauma in the foot-knee-back department. The longitudinal branches of the strengthener make it possible to orientate the unfurling of the foot and avoid these eversions or inversions of the front of the foot, and furthermore give rise to a xe2x80x9cspoonxe2x80x9d effect which encourages propulsion.
To encourage this xe2x80x9cspoonxe2x80x9d effect during propulsion, the branch located on the inside of the sole extends forward beyond the first metatarsal head.
According to another feature of the invention, the central zone of the strengthener located forward of the zone on which the heel rests is raised relative to the zones arranged in front of and behind it.
This structure restrains the heel in its forward tilting movement and actually affords a spring effect during the phase of the step serving as a complement of shock-absorbing to the arches and as additional energy restitution to the xe2x80x9cspoonxe2x80x9d effect during the propulsion phase.
Advantageously, in its central zone located forward of the zone on which the heel rests, the strengthener has two lateral recesses, the far ends of which are set back from the respective lateral edges of the sole.
By virtue of these recesses, the flexible zones of the foot, or arches, are uncovered, which is in keeping with their dynamic functions of flexibility to provide shock-absorbing and restore energy needed for the unfurling of the step.
According to another feature of this sole, the main part of the sole comprises, in the zone on which the heel rests and/or in the zone that lies between the two branches of the strengthener, an upwards facing bulge corresponding to a downwards facing concave hollow. This is a double concavity in the transverse direction and in the longitudinal direction. When the body weight is applied to one or other of these two bulged zones, vertical force tends to tighten the loop in the case of the rear bulge, or pull the longitudinal branches together in the case of the front bulge, thus improving the stabilizing effect.
Advantageously, in order to enjoy a good rigidity/responsiveness ratio, the strengthener is made of a synthetic material or of a composite material chosen from polymers, polycarbonates, polycarbolactols or carbon- or glass-fibre-reinforced resin.
According to a first embodiment, the strengthener is made as a single piece obtained by cutting out.
According to another embodiment, the strengthener is made from an elongate single piece bent to form a loop and the ends of which form longitudinal branches after crossing in the zone located forward of the heel. The crossing of the two branches forward of the zone that takes the heel forms the desired extra thickness for the heel, and whose function was defined earlier.
Furthermore, in the crossing zone the two branches are secured together or attached in an elastic and adjustable fashion.
According to another embodiment, the strengthener is made in two parts of which one forms the loop surrounding the heel and the branch located on the inside of the sole and of which the other, fixed to the first part on the inside of the sole and in the zone forward of the loop formed by the strengthener, extends essentially transversely with respect to the axis of the sole to meet the outside edge of the sole and form the second longitudinal branch.
Advantageously in this case, the two constituent parts of the strengthener are made of different materials.
The second part, constituting the longitudinal branch extending along the outside of the sole, has an inclined part which forms an angle of between 60 and 90xc2x0 with the axis known as the metatarsal break, which is the line along which the front of the foot bends. This orientation of the oblique part of the branch allows physiological tilting of the foot from the lateral to the medial, which makes it possible to accelerate the way in which the pressure points unfurl and the phase of slowing the front of the foot, and optimum unfurling of the pressure points to arrive at a pressure point on the big toe, by which the impetus is given.
According to one possibility, the ends of the branches of the strengthener are connected by a bridge, which makes it possible to transfer the body weight more easily from one branch to the other if the weight is bearing too heavily on one of the branches. In order to achieve or improve this realignment effect, at least one of the branches of the strengthener is equipped with a localized elastic pad or spring.